Position or displacement sensing devices using optical techniques are known from German patent document (Patent Application-Auslegeschrift) No. 1,962,515 and from British patent document (Published Application) No. 2,077,421.
The German patent document No. 1,962,515 describes an optical distance sensor of the contactless type in which a luminous beam is focused to a plurality of distinct foci. This sensor determines the position of an object which is located between two such foci. In this system, the respective intensities of two light wavelengths reflected by the object and converging at these foci are compared and the position of the object with respect to the sensor is defined when these wavelengths are of equal intensity. The sensor is displaced to obtain such equality. The final position of the sensor fixes the location of the object. The sensor is in effect a hybrid sensor which comprises a mechanical measuring system for establishing its location in space and an optical measuring system for locating the element with respect to the sensor.
The British patent document describes an optical detector and a process for measuring the displacement of an object in which two monochromatic beams of different colors but equal intensities are focused along a common axis. This yields two distinct focal points situated at equal distances from a place of reference. The relative intensities of the respective beams after reflection by the object can then be measured. This relative intensity can be a difference or quotient of the intensities of the light of the two wavelengths and the change in the value of this relative measurement is a function of the displacement of the object from which the two monochromatic rays are reflected.
In both cases, therefore, it is necessary to compare two optical signals and the precision of the measurement and hence of the comparison is a function of the intensities of these signal. As a consequence, the precision and resolving power of the detectors are clearly connected to the optical properties of the surface of the object whose location or position is to be measured and, specifically, the ability of the object to reflect adequate amounts of the different incident light beams. This system is also sensitive to nonlinear variations in light intensity which may cause a disproportionate intensity for two wavelengths upon any variation of source intensity.